Microwave photonic links require linear optical transmitters in order to achieve high dynamic range (typically characterized by the Spurious Free Dynamic Range or SFDR). Current optical transmitters based on directly modulated diode lasers or external modulators such as Mach-Zehnder modulators and EA (Electroabsorption) modulators, employ elaborate feedforward as well as feedback linearization techniques in order to suppress signal harmonics due to the non-linearity of diode lasers (in the case of direct modulation) and that of external modulators. Examples of such techniques are shown in FIGS. 1 and 2.
FIG. 1 illustrates an example of a feedforward dual external modulators technique. In FIG. 1, the output of diode laser 105 is coupled, by coupler 110, to an upper branch that includes polarization controller 121 and external modulator 1, which may conventionally be, for example, a mach zender modulator or an electroabsorption modulator. The modulator 1 receives an RF modulating input and bias control from a modulator bias control circuit 125. The diode laser output is also coupled, by the coupler 110, to a lower branch that includes polarization controller 131 and external modulator 2, which, again, may conventionally be a respective mach zender modulator or an electroabsorption modulator. The modulator 2 receives the RF modulating input via variable gain circuit 133, and bias control from a modulator bias control circuit 135. The outputs of the external modulators are combined by coupler 140 to produce the linearized optical output. Appropriate fiber delays are also represented in the diagram.
FIG. 2 illustrates an example of a feedforward direct modulation technique. In FIG. 2, an electrical input, such as an RF input, is split by a microwave splitter 255, one output of which is coupled to a diode laser 260, and the other output of which is coupled, via an electrical delay, to the positive input of a 180 degree hybrid coupler 270. The optical output of diode laser 260 is coupled by coupler 262 to a photodetector 275, the electrical output of which is, in turn, coupled via variable gain circuit 277, to the negative input terminal of 180 degree hybrid coupler 270. The output of coupler 270 is coupled, via an electrical delay, to another variable gain circuit 278 whose output is coupled to another diode laser 280. Another optical coupler 263 receives the output of diode laser 280 and also receives, via a fiber delay, the output of diode laser 260, and the coupler 263 combines these optical signals to produce the desired linearized optical output.
FIG. 3 illustrates an example of a feedback linearization technique. A diode laser 310, under control of a bias control circuit 305, produces an optical output that is coupled to an external modulator 320, for example of the mach-zehnder type. In this illustration, the modulator 320 receives an RF input, which may be predistorted. The optical output of modulator 320 is coupled, by optical coupler 330, to photodiode 340, whose output controls a modulator bias control circuit 350 which, in turn, provides bias control to the modulator 320. The feedback controlled modulator output is the linearized optical output.
Regarding feedback linearization, as is evident in FIG. 3, feedback is provided by means of an external monitor photodiode. The signal is then used as an input to a control circuit which compensates for the distortion inherent in the diode laser output. This results in the need for couplers, and introduces unnecessary coupling losses in the feedback loop.
Feedback linearization techniques are common in optical sources based on directly modulated diode lasers or external modulators, such as the Mach-Zehnder modulator of FIG. 3. Feedback techniques are able to improve the SFDR to 100 dB/Hz2/3 and above. (see, for example, Emcore data sheet, “Small Integrated Transmitter Unit SITU2400-3000”, October 2007). However, linearization techniques based on diode lasers, due to its two-terminal limitation, requires a complex assembly of both active and passive components (for example, greater than 8 components per module) and consumes high power, for example in excess of 2 Watts. These solutions can be prohibitively expensive (for example, greater than $10K per transmitter module) and lack integration capability. In some cases, feedback is used in conjunction with predistortion technique to achieve better bandwidth performance. This again results in higher costs, complexity and power consumption.
It is among the objects of the invention to further improve feedback linearization techniques and systems.